Just adding to info already given: if your are your father's durable PoA (and you have the actual, original paperwork that was signed and notarized) AND if your father has an actual medical diagnosis by a doctor of dementia (which would be in his medical records) your father can no longer change the PoA. Your brother would have to pursue guardianship through the courts.
If yes to the above conditions, then do as FloridaDD below said: lock down and protect all your father's sensitive financial info (including passwords to any devices he may own), put all his banking online, get possession of the titles of things (like car, house), chechbooks (especially!), etc. Maybe have a discussion with a banker to tell them the situation (and go with the PoA paperwork). If your brother starts having his mail and bills as your father's address, this will require him to be evicted, a 30-day process that requires filling out a form and paying ($300-ish depending on his state) and then posting at the residence. But you should act before brother gets "settled in". You may be able to call the police before he settles in, but after that it becomes a civil matter and the cops have no power at that point.
If your father does not have cognitive decline, then this new arrangement is what he wants, and he can have it. What are you concerned about with your brother?
If your father has a sound mind he can make this change. If he has dementia it wouldn’t be right for it to happen, but it’s still been known to. Talk to dad, away from brother, and tell him your concerns. Does he also have an up to date will and advance directive? Those are also important documents that need to be in place.
Do you have both medical and financial POA? And if POA is "springing" has it been declared operative (i.e, has doctor or doctors, as required, diagnosed dad as lacking cognitive ability)? If so, first have all bills and all financial information sent to your address. Immediately. If dad cannot drive, sell or give away car. Change PIN on ATM. If you are POA, and not in best interest of dad to live there, contact lawyer re eviction If brother has just moved in, it may be easier. But get him out, change locks
If yes to the above conditions, then do as FloridaDD below said: lock down and protect all your father's sensitive financial info (including passwords to any devices he may own), put all his banking online, get possession of the titles of things (like car, house), chechbooks (especially!), etc. Maybe have a discussion with a banker to tell them the situation (and go with the PoA paperwork). If your brother starts having his mail and bills as your father's address, this will require him to be evicted, a 30-day process that requires filling out a form and paying ($300-ish depending on his state) and then posting at the residence. But you should act before brother gets "settled in". You may be able to call the police before he settles in, but after that it becomes a civil matter and the cops have no power at that point.
If your father does not have cognitive decline, then this new arrangement is what he wants, and he can have it. What are you concerned about with your brother?